Two-quasiproton states in 168Er studied by the 169Tm(t, α)168Er reaction

The 169Tm(t, α)168Er reaction has been studied using 17 MeV polarized tritons from the Los Alamos National Laboratory tandem Van de Graaff accelerator. The α-spectra were analyzed with a Q3D magnetic spectrometer. The overall energy resolution was typically ~ 15 keV (FHWM) and angular distributions of cross sections and analyzing powers were obtained for levels up to ~ 2.7 MeV. The fact that spins and parities for all levels up to ⩾ 2 MeV were previously known from an extensive series of (n, γ) studies made it possible to determine specific two-quasiproton structures for many bands from the present results. The Kπ = 2+ γ-vibrational band was found to have a large 32+ [411]p + 12+[411]p admixture, consistent with the predicted microscopic composition of this phonon, but no 52[413]p − 12+ [411]p component was observed. The Kπ = 04+ band at 1833 keV has ∼ 25% of the 12+ [411]p − 12+[411]p two-quasiproton strength. This is in excellent agreement with the Soloviev model but is inconsistent with the interacting boson model, in which the Kπ = 04+ band is composed almost completely of multiphonon configurations that should not be populated in a single-nucleon transfer reaction. The Kπ = 4−, 72−[523]p + 12+ [411]p two-quasiproton and the Kπ = 4−, 72+[633]n + 12−[521]n two-quasineutron states are mixed strongly with each other, but the two Kπ = 3− bands composed of antiparallel couplings of the same particles are not. A good qualitative explanation of this mixing pattern is provided in terms of the effective neutron-proton interaction.